---
title: "Brick masonry infill walls in RC frames"
subject: "Concrete Technology and Masonry Structure"
chapter: 7
key: "semester_5/concrete_technology_and_masonry_structure:masonry-infill-walls"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/masonry-infill-walls
site: BCT Engineering
---

# Brick masonry infill walls in RC frames

Concrete Technology and Masonry Structure · Chapter 7: Introduction to masonry structures

- Asked in: 2082 Bhadra, 3 marks; 2081 Bhadra, 4 marks; 2074 Chaitra, 4 marks (asked 3 times)
- Video: [Masonry Infill Walls and Damping - Lecture 9 | Structural Assistant | MD Assistant Studio](https://www.youtube.com/watch?v=R1TScA-iKWs) — Vi-Harsh Vibes
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/masonry-infill-walls
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-7

## Question

Describe the role of brick masonry infill walls in RC frame structures (effect of masonry infill in a frame structure, with advantages and disadvantages), with neat sketches.

## Short answer

Infill walls act as diagonal compression struts, greatly increasing frame stiffness and lateral strength, reducing drift. Advantages include energy dissipation and insulation; disadvantages are soft-storey formation, short-column effect, out-of-plane failure, and added seismic mass.

## Answer

**Masonry infill** is brick or block masonry built within an RC (or steel) frame to form partitions and enclosure. It is usually considered non-structural and ignored in design, but it strongly changes the behaviour of the frame under lateral loads.

#### Role and effect on frame behaviour
- Under lateral load the infill acts as a **diagonal compression strut** between the frame corners, so stiffness and lateral strength rise and storey drift is reduced.
- The initial stiffness is 5 to 10 times that of a bare frame; the fundamental period is reduced, so the seismic force attracted rises.
- Cracking and crushing of the infill dissipate energy.

```text
  lateral    +------------+
  load  -->  |\           |
             |  \         |
             |    \       |    infill acts as a
             |      \     |    diagonal compression strut
             |        \   |    (equivalent width w)
             |          \ |
             +------------+
              frame + infill
```

Equivalent strut width (Mainstone): $w \approx 0.175\,\alpha_h^{-0.4} D$, where $D$ is the diagonal length of the infill and $\alpha_h$ measures the stiffness of the infill relative to the frame.

#### Advantages
1. Increases the stiffness, strength and energy dissipation of the frame.
2. Reduces drift and damage to non-structural items in low-intensity shaking.
3. Gives thermal and sound insulation, fire resistance and enclosure, at low cost with local materials.

#### Disadvantages
1. Irregular or non-uniform placement (open ground storey) creates a **soft storey** and torsion.
2. Partial-height infill leads to the **short column** effect and shear failure of columns.
3. It is brittle and prone to **out-of-plane failure** if not tied to the frame (it needs bands, ties and mesh as per NBC 105 and IS 4326).
4. It adds mass and so increases the seismic force; heavy walls need strong frames.
5. Diagonal strut action may overload the column and joint.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. How does masonry infill act under lateral load?
   A. As a tension tie that makes the frame flexible
   B. It does not interact with the frame
   C. It acts as a hinge that reduces stiffness
   D. As a diagonal compression strut between frame corners, raising stiffness and strength and reducing drift
   Answer: D. The strut model (Mainstone, w = 0.175 alpha^-0.4 D) represents infill. (Key point: Infill strut action)

2. What is the effect of infill on period and seismic force?
   A. Initial stiffness is 5 to 10 times bare frame, period is reduced, so seismic force attracted rises
   B. Period increases so seismic force falls
   C. Stiffness is unchanged
   D. Seismic force is zero
   Answer: A. A stiffer structure has a shorter period and attracts more force. (Key point: Seismic effect of infill)

3. What problem arises from open ground storey (no infill below)?
   A. Short column effect
   B. Soft storey and torsion
   C. Increased stiffness at base
   D. Out-of-plane failure only
   Answer: B. Non-uniform infill creates a weak storey where damage concentrates. (Key point: Soft storey)

4. How is out-of-plane failure of infill reduced?
   A. Make it thicker without ties
   B. Leave a gap all around the wall
   C. Use weaker mortar
   D. Tie it to the frame with bands, ties and mesh (NBC 105, IS 4326)
   Answer: D. Brittle infill that is not tied can fall out of the frame. (Key point: Out-of-plane failure)

## Related in Concrete Technology and Masonry Structure

- [Uses and scope of masonry structures](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Amasonry-uses-scope) — Concrete Technology and Masonry Structure, chapter 7, asked 5×
- [Use of masonry; types of brick bond](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Amasonry-uses-bond-types) — Concrete Technology and Masonry Structure, chapter 7, asked 2×
- [Advantages of masonry structures](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Amasonry-advantages) — Concrete Technology and Masonry Structure, chapter 7
- [Flat-jack test for brick masonry wall](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aflat-jack-test) — Concrete Technology and Masonry Structure, chapter 10, asked 2×
- [Brick masonry construction techniques](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Amasonry-construction-techniques) — Concrete Technology and Masonry Structure, chapter 7
- [Closers in brick masonry; English and Flemish bond](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aclosers-english-flemish-bond) — Concrete Technology and Masonry Structure, chapter 7

## Related in other subjects

- [Distribution of lateral force among masonry walls](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Alateral-load-sharing-among-piers) — Design of Timber and Masonry Structures, chapter 6
- [Slab on masonry walls, 5x6 m](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Aslab-on-masonry-walls-5x6-m) — Design of RCC Structure, chapter 9
- [In-plane and out-of-plane behaviour of masonry walls](https://www.bctengineer.com/question.md?key=semester_5%2Fdesign_of_timber_and_masonry_structures%3Ain-plane-out-of-plane-behavior) — Design of Timber and Masonry Structures, chapter 6
- [Design of 250x300 beam on masonry walls](https://www.bctengineer.com/question.md?key=semester_7%2Fdesign_of_rcc_structure%3Adesign-of-250x300-beam-on-masonry) — Design of RCC Structure, chapter 4